This invention relates to a solid state image pickup apparatus employing a frame interline transfer type solid state image sensor and, more particularly, to a solid state image apparatus having the function of a so-called electronic shutter.
A solid state image sensor such as CCD (charge coupled device) image sensor, has advantageous features such that it is free of burning or residual images as compared to an image pickup tube device, it is highly shock-proof, and lends itself to miniaturization of video cameras. In video cameras conforming to the standard television systems, such as NTSC systems employing an interlace scanning system, a CCD image sensor is employed in which image signals from pixels of odd-numbered fields and those from pixels of even-numbered fields are read out alternately at intervals of one-field periods (one-sixtieth of a second in the case of the NTSC system).
The solid state image sensor, such as the above-described CCD image sensor, may be classified, according to the type of the transfer structure used for outputting image pickup electrical charges, into a frame transfer type, interline transfer type and a frame interline transfer type. In the interline transfer type CCD image sensor, for example, one-field image pickup charges, produced by the photosensitive elements, are transferred at intervals of one-field periods to vertical transfer registers by sensor gate pulses so as to be read out line-sequentially from the vertical transfer register via horizontal transfer registers.
On the other hand a so-called progressive scan CCD or PS CCD image sensor, in which a horizontal transfer gate for two horizontal lines is provided so that image pickup signals from pixels of odd-numbered fields and those from pixels of even-numbered fields are read out in their entirety during a one-field period, has been presented for use in high-resolution video cameras, such as in high-vision television systems.
Besides, there is also presented a CCD image sensor of the type having the function of an electronic shutter for controlling effective charge storage periods of the photosensitive elements electronically. With the CCD image sensor having the function of the electronic shutter, a shutter control pulse is applied to its substrate for sweeping out image pickup charges of the photosensitive elements in an overflow drain for controlling the light exposure time, that is the effective charge storage period, as shown in FIG. 4. In this figure, a shutter control pulse .phi.SP.sub.1/1000 which will give an effective charge storage period T.sub.1/1000 of a duration of 1/1000 second, and a shutter control pulse .phi.SP.sub.1/700 which will give an effective charge storage period of a duration of 1/700 second, are shown. Time intervals beginning at t.sub.1 and t.sub.2 and ending at t.sub.0, the time of the first one of the sensor gate pulses .phi.SG, yield effective storage periods T.sub.1/1000 and T.sub.1/700, respectively.
In the conventional CCD image sensor, having the function of the electronic shutter, the end timing of the effective charge storage period or the effective exposure period is the timing .+-. of the sensor gate pulse .phi.SG, such that the end timing of the effective light exposure period is constant for any shutter speed, with the start timing of the effective exposure period being changed with the shutter speeds. Conversely, with shutter control of a mechanical optical shutter in an optical camera employing a halide film, light exposure is started on actuation of a shutter release so that light exposure is terminated at a timing associated with the shutter speed.
The electronic shutter control in the CCD image sensor is related in an inverse fashion with the shutter control in the optical shutter in an optical camera, so that, in a solid state image pickup device employing the CCD image sensor having the function of the electronic shutter for photographing still images, the start timing of the effective light exposure period tends to be delayed with respect to the shutter release operating timing, so that the opportunity of shutter actuation tends to be lost. In industrial application, if it is attempted to actuate the function of the electronic shutter in photographing an object conveyed on a transfer line, the start timing of the effective light exposure period is delayed, thus rendering it difficult to photograph the object reliably.
Besides, with the electronic shutter function in the above-described CCD image sensor, since shutter control is made by applying a shutter control pulse to its substrate, a noise known as a shutter step (step-like fluctuations in substrate voltage caused by electronic shutter operation in each horizontal blanking period) is produced in a known manner at a boundary of the shutter control pulse. Such deterioration in the picture quality is also produced by shutter step when the CCD image sensor is used in a video camera.
The U.S. Pat. No. 4,984,002, assigned to the present assignee, has been proposed for overcoming the aforementioned problem. In accordance with this prior-art system, the time which elapses between application of a trigger signal until the next vertical synchronization signal is issued is controlled for maintaining the constant start timing of the effective exposure interval.
It is noted that the video camera proposed in accordance with the present invention is frequently put to practical application, in which case it is frequently desired to supply external synchronization signals to a video camera for synchronizing video signals thereto.
With the system of the U.S. Pat. No. 4,984,002, since the timing of the next vertical synchronization signal is determined by the timing of the trigger signal, video signals synchronized to the external synchronization signals can not be produced.